It does pretty much steam in a slow cooker so you get a more poached effect. Rather than use foil an idea is to sit your chicken on chunks of onion or chunky veg and this will make for a tasty stock which you can make gravy or soup with .

I wouldn't use any water at all. Frankly with wet heat from the juices, I don't think a whole chicken is suited for a crock pot. A whole roasted chicken in the oven is so easy to do with spectacular results and only takes an hour, or maybe a little more.